Effects of Lithology on Asymbiotic N2 Fixation in Subtropical Secondary Forests, Southwest China

Asymbiotic N-2 fixation (ANF) is a major nitrogen (N) input pathway to terrestrial ecosystems. However, there is considerable uncertainty about how lithology affects ANF. Here, we measured ANF rates in soil, litter, and moss in forests underlain by limestone (limestone forest), dolomite (dolomite forest), and clasolite (clasolite forest), respectively, in southwest China. Effects of lithology on soil ANF rate varied seasonally. The rate was highest in the dolomite forest during the wet season, but was highest in the limestone forest during the dry season. The overall soil ANF rate was significantly higher in the limestone forest than in the clasolite forest. Litter ANF rate was significantly higher in the limestone forest and dolomite forest than in the clasolite forest regardless of season. There was no significant difference in moss ANF rate among the three types of forests in both seasons. The annual N-2 fixation rate was highest in the limestone forest (1.72 +/- 0.27 kg N ha(-1) y(-1)) but lowest in the clasolite forest (0.70 +/- 0.08 kg N ha(-1) y(-1)). In the wet season, the variation of ANF rates was best explained by soil nitrate and available iron for soil, and by litter N and calcium content for litter. In the dry season, the variation of ANF rates was best explained by soil water content, ammonia and total phosphorus for soil, and by litter water content and calcium content for litter. No strong explanatory variables were identified for ANF in moss during the wet or dry season. Our findings suggest that lithology significantly affected ANF in soil and litter, but not in moss, and hence should be considered in Earth system models to facilitate better prediction of N inputs via biological N-2 fixation under global change.